Ah windows file sharing. So many things that can go wrong:
If windows explorer is previewing the file for the stupid little preview thing then it will lock the file. This is bad.
Make sure that the samba user you are logged in as owns the file and has rw permissions in *nix.
If the group execute bit is set, make sure you don't have 'map system' set 'yes' in your samba config. Actually I'm not sure that's the correct bit (map system, I mean), but all your map stuff should be set to 'no', the default, unless you are sure you want it set otherwise.
Sharing your home folder is excellent. Particularily if you use the [homes] facility since it is so easy. If you continue to have problems, turn your log level up to 2 and see exactly what permissions are being granted on that file.